GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Venting and Dehumidifying a Crawlspace

arthur_roberts | Posted in General Questions on

I have a 1920’s home in Berkeley, CA with a 1,200 sq ft crawlspace that has only one 6″ x 8″ vent.  I’m wondering if I should close the vent and run a dehumidifier, or add another vent.

Relevant facts:

I’m in the process of air sealing and insulating the rim joists and ceiling of the crawlspace, so it will get colder down there.  Relative humidity hovers around 60s and sometimes low 70s.

Rat slab on the ground.  I’ll eventually put in a vapor barrier, but that may be another couple years due to other projects.

We have a boiler with cast iron radiators for heat, but no radiators in the crawlspace (of course) and no easy way to condition the air.

I’m going to put in the dehumidifer no matter what, unless someone tells me that’s a bad idea.  My main question is whether I add a vent, or close up the existing one, or just leave as is.  If there are other options I’m not thinking of, please share!

Thanks so much!  This community is amazing btw.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. mr_reference_Hugh | | #1

    There was a similar question here about a week ago.

    Here is an article that talks about dealing with humidity in crawl spaces, among other issues with CS. GBA article by Allison A. Bailes III, PhD.
    "Five Ways to Deal with Crawlspace Air"

    I will also provide links to two older posts here and a search on a bunch of GBA articles.

  2. Deleted | | #2


  3. arthur_roberts | | #3

    That article is great, thank you! I think I may seal the vent after I install the dehumidifer, which is option 4 from the article.

  4. walta100 | | #4

    Seems to me your house has found a balance for moisture in and out as is and has stood the test of time any change you make you risk upsetting that balance.

    Yes, today we would we would not likely build a vented crawlspace but I see nothing in your plan that seems likely to lower your fuel usage in fact adding the dehumidifier seems likely to increase your usage. If your plan was to air seal an add perimeter insulation and condition the crawlspace of a house in a cold climate, I could see some saving.


  5. user-5946022 | | #5

    If you add a dehumidifier to the crawlspace, you will need to seal the vent, and the entire crawl, otherwise you are attempting to dehumidify the outdoors.

    However, walta100 has a good point. What is is you are hoping to achieve by adding the dehumidification to your crawl space?

  6. arthur_roberts | | #6

    My relative humidity sometimes goes up to 70%. Isn't that too high? That's the issue I'm trying to solve. Especially now that the temperatures are a little lower down there due to the new insulation, I don't want to have mold issues.

    1. mr_reference_Hugh | | #7

      My own point of view is that you have made some changes to the "building system" and you see that there has been an impact (temperature and humidity). You are looking to mitigate the risk. It all sounds reasonable to me. Looking forward to see other comments.

  7. walta100 | | #8

    Please tell us about the insulation you added type, R value and location.


    1. arthur_roberts | | #9

      Hi Walta, the insulation is R-21 faced fiber glass batts on the ceiling of the crawlspace, with face upwards toward conditioned space. Rigid foam on the rim joist.

      1. seabornman | | #10

        One reason I like to see the foundation walls insulated and not the floor is that mice seem to enjoy living in fiberglass insulation. I have had bad personal experiences removing mice-infested horizontal insulation.

      2. user-5946022 | | #11

        Typically, you only install insulation between a conditioned and an unconditioned space.

        Your first post indicates you want to dehumidify the crawl space. You need to seal up your open vents before doing that otherwise your dehumidifier will run forever trying to dehumidify the outside air that is always exchanging through the open vents with the crawl space air. Once sealed up - seal all perimeter vents and perimeter penetration, and seal up the ground if it is exposed dirt, etc. It is also a good idea to insulate the perimeter of your crawl. Then dehumidify - you are essentially bringing your crawl into the conditioned envelope.

        Then you posted that you have insulated the floor system between the crawl and the house. That indicates you intend for the barrier between your conditioned and unconditioned space to be that floor system. In that case, seal up all the holes in the floor system and let the crawl be unconditioned. You might need to add some vents for good airflow.

        HOWEVER - where are your HVAC ducts? You generally want those in a conditioned space.

        What caused you to insulate the underside of the floor of your living space? What issue were you trying to resolve? If you want to get rid of cold floors and stop drafts, encapsulating the crawl generally produces far better results, and there is less area to insulate by insulating the crawl space walls. Fortunately, if the underside of the floor is insulated with fiberglass batts, they are inexpensive and easy to remove. The rim joist insulation would stay. It is not really doing anything for an unconditioned crawl, but is needed for a conditioned crawl. You will also need to air seal at the rim joist if you want a conditioned crawl.

        1. arthur_roberts | | #12

          I don't have HVAC -- just boiler with cast iron radiators, so didn't have the ability to heat up the crawlspace. That's why I put the insulation in the ceiling of the crawl. I put the foam board on the rim joists to prevent condensation occurring there.

          What would you recommend given no HVAC and inability to heat the crawl? My plan is to seal the crawl then add the vapor barrier to the floors/wall, and add dehumidifier. But would you do something different?

          1. user-5946022 | | #17

            You have HVAC (it stands for heating, ventilation & air conditioning).
            Your boiler is the H part of your HVAC, delivering hot water to the radiators.
            You don't need to heat a crawl space - I've actually never heard of that. A conditioned crawl is nothing more than the insulation is at the perimeter of the crawl and the crawl is sealed, so you don't care if conditioned air from the house gets into the crawl - it's still not going to escape to the outdoors.

            As for what I would do, again, what problem are you trying to solve? What caused you to insulate the underside of your living area?
            Comfort - ie cold drafts from crawl? Cold floor? Other?

  8. walta100 | | #13

    The way I see it there are 3 kinds of crawlspaces.

    1 Vented this work great but is an energy pig.

    2 Conditioned the owner is committed to buying and operating the equipment required to keep the crawlspace at more or less the same temp and humidity as the rest of the house.

    3 The inbetweeners these people are looking for the free lunch and are willing to risk rot and mold to get it. Note it is a code violation.

    The fact that you spent money insulating both the floor and the walls leads me to believe your plan is in between. It has been my experience that this group is not persuadable so I will stop now.

    Note a dehumidifier is an electric heater! 100% of the electric it uses is converted to heat and added to the crawlspace.


    1. arthur_roberts | | #14

      Thanks for the input thus far. It's true that it's somewhat of an "in between" option due to the lack of HVAC. I know you said you're stopping, but to avoid damaging my house, I'd love to hear about why there's a risk of rot and mold with my approach. I thought the dehumidifier would mitigate that risk, but what am I not thinking of?

  9. frasca | | #15

    70% RH doesn't matter much in NorCal; your absolute humidity is quite low year-round. Given your climate and the fact that you're heating-only, the only moist air source your crawlspace framing might encounter is the indoor air that the occupants generate in the winter, and the stack effect will have that air tending to going up, not down.

    With your R21 batts and the kraft paper facing upward I don't see any reasonable likelihood of moisture issues in that crawl and I don't see the point of a dehumidifier.

    As others have noted I think you would have been better off NOT insulating the crawlspace ceiling, but rather sealing the crawl from the exterior, insulating the stem walls, and doing whatever you wanted on the floor. I doubt that a rat slab works well as a vapor retarder, but I don't know how much the vapor permeance matters if you are on dry soil. Though the energy benefit is of this type of crawlspace is small in NorCal - as with most things given the low HVAC needs of your climate - it is just a lot more pleasant to store stuff in and crawl around in when you're rewiring your home theater or plumbing your new espresso machine.

    But the pain of pulling out those batts now would be a lot to bear, so if you are reasonably confident that the ceiling is well air-sealed and the batts aren't going anywhere, you might as well commit to the vented crawlspace.

    As to your question of whether to add a second vent; I predict it would add some marginal amount of passive airflow to your crawlspace; maybe helping to dry the framing on the off chance that the framing was ever damp, but also adding some small amount of wind-washing to your batt insulation.

    I'd probably pass and just observe for a year to see if I ever saw damp framing.

  10. walta100 | | #16

    I have to admit some people do get the free lunch in that the amount of moisture that enters their crawlspace is small enough that the dew point of the air in their crawlspace happens to stay above the surface temp of the walls of the crawlspace. Random luck of the draw can’t be calculated or guaranteed to work and it still a code violation.

    The fact that you are considering a dehumidifier tell me you’re not firmly in the free lunch camp.
    While the dehumidifier is removing some moisture it does while adding heat to the space. In the end you will heat the space and rising the temp of the air keeping the walls warm enough no water will condense on them.

    You say you have a boiler. I have never heard of an undersized boiler so you could add another zone to condition your crawlspace to the point that it is more or lees the same temp and humidity as the rest of your home. I would likley cost less to operate and not fail after after a year or so the way dehumidifiers tend to.


    1. Expert Member
      DCcontrarian | | #21

      Heating a crawlspace isn't going to bring the humidity in line with the rest of the house unless there is air exchange. Absent some way of removing humidity the heat will just accelerate mold and rot growth.

  11. Expert Member
    DCcontrarian | | #18

    I always recommend this article for crawl spaces:

    Here's what I'm thinking:

    Whenever you have an assembly (generic term for wall, floor or roof) that uses absorbent materials you want it to have a way for it to dry. In general, moisture tends to flow from wet to dry and from warm to cool. In most places at most times one side of the wall is both warmer and wetter than the other side, so the moisture drive is from the warm wet side to the cool, dry side. The standard way to build an assembly is to put a vapor barrier on the warm, wet side, to keep as much moisture out of the assembly as possible, and then construct it so that moisture can dry to the cool, dry side.

    If you try to put an assembly in a floor above a crawl space, you run into the problem that the crawl space is going to be cooler and wetter than the interior of the house so there is vapor drive in both directions. I've only ever lived east of the Mississippi, but everywhere I've lived a crawl space will stay at about 55F and 99% humidity year-round. An assembly like the one you've built will have water dripping off of the bottom of the joists in summer. The insulation keeps any heat from getting down there, and the outside dew point is in the 60's so any outside air that gets in immediately condenses. In the winter there is no drying either, a dirt floor acts as essentially an unlimited reservoir of moisture keeping the humidity high and negating any moisture drive from the house being warmer than the crawlspace.

    There are two approaches to dealing with this situation. One is to make the floor a non-absorbent assembly: encapsulate the whole thing in closed cell foam. The other is to make your insulating assembly the walls of the crawl space rather than the ceiling/floor. If the walls are sealed and insulated well and there's any airflow from the living space the crawlspace will pretty much have the same conditions as the interior. If you're in a cooling-dominated climate your walls are drying to the interior and you can have your crawlspace walls dry to the interior. In a heating-dominated climate your walls are drying to the exterior, which isn't going to happen in the crawlspace so you want to use non-absorbent insulation and let your drying happen above-ground.

    In either case a key part is to have a vapor barrier on the crawl space floor, in my climate trying to dry the earth under a house is like drying the ocean.

    The reason we're having trouble with your assembly is that it's not the way things are normally done. Will it work? I don't know, maybe. I know it wouldn't work in my climate, it might work in yours. If it does work, the way that it will work is that the dehumidifier will keep the crawlspace cooler and dryer than the interior of the house, so that any moisture that gets into the floor assembly will dry to below and be removed by the dehumidifier. In order for that to work, you want the crawl space to be as tight as possible, your enemy is humidity brought in by outside air.

    And you really want to cover that dirt floor. I don't understand at all the statement, "I’ll eventually put in a vapor barrier, but that may be another couple years due to other projects." All you need is a sheet of plastic. Go to Home Depot and get the largest sheet they have. It will be about $140 and it will take you maybe half an hour if you're slow to spread it in the crawl space. Less time than you've already spent on this thread.

    Down the road, paint the concrete floor with a vapor barrier primer that's rated for concrete.

  12. mr_reference_Hugh | | #19

    Mr. Roberts,

    I don’t want to tell you what to do but I share what I would do and share a Building Science article.

    I agree with others on certain points but still think that the dehumidifier would be a good idea at this point and that means closing the vents.

    In the future you already intend to place a class 1 vapour retarder (vapour barrier) on the floor (others suggested doing it now if possible).

    In the future, I would be air sealing the crawl space from the outside using air sealing techniques discussed on GBA.

    I found this article from Building Science Corp. that makes a lot of sense to me and it’s main point is to get rid of vents once you start to enclose a crawl space. An extract of the pertinent part of the article is shown below:

    Crawlspaces are real simple to understand and deal with. When you vent crawlspaces you bring in hot, humid air and cause moisture and mold problems. The ground surface is typically colder than the dew point temperature of the exterior air. The underside of crawlspace floor insulation is radiation coupled to the ground surface and is very close to the same temperature of the ground. Moisture droplets can be seen all over the top surface of typical polyethylene ground covers as well as hanging from the bottom surface of the crawlspace floor insulation. Gee, I wonder how all the water got through the poly ground cover? It must have leaked through the walls. Give me another break. Now, when the moisture is in the insulation where do you think it wants to go? Where is the air conditioning? Moisture moves to the cold surface. Venting crawlspaces made sense only when you had no air conditioning and no insulation and no crawlspace walls.

    This article is about building in the south but you mentioned that the crawl space is cooler and we know the the ground is always cooler when not exposed to sunlight.

    I am in climate zone 7 but we have hot humid summers and I would expect to experience the same issues here as in the south during the summer.

    If you have a walled-in crawl space, I think you really need to think about conditioning that space. If you can get it air-sealed in the future, that would be key…in addition to the vapour retarder. But I would leave the dehumidifier in place even after the work planned, or at a minimum to monitor closely the humidity in the crawl space and pull back a batt of insulation every three months for a year or two.

    The air sealing on the perimeter would eliminate exterior humid air from entering the crawl space. The degree to which you air seal will determine how much humid air will continue to enter the crawl space.

    I would not worry too much about insulating the perimeter because you are not trying to heat the crawl space. Of course, the energy used by the dehumidifier to do its work will create a small amount of heat.

  13. frasca | | #20

    I generally prefer unvented, conditioned crawlspaces for many reasons, including energy, accessibility, and health of the framing. However I am still unconvinced that vented crawlspaces are a major source of moisture problems on the Pacific coast.

    I pulled the last 20 years of dew points and dry bulb temps from for Oakland International Airport, which is as close as I can get to Berkeley, and after some data translations built the attached chart.

    Our friend Arthur is very unlikely to see outside air with a dewpoint above 57deg F, and when he does the dry bulb temp will almost certainly be >65deg F.

    What temp is the framing? Even ignoring heat transmitted from the living space above I can't even really imagine the ground temp being that far below 57deg F... I've heard that a few feet underground it's generally about 55deg. So the outside air is not likely to present a condensation problem IMO...

    That leaves 2 other sources of moisture that I can think of: the living space and the soil. Again, the air in the living space is generally going up due to stack effect, and Arthur is doing what he can to air-seal the crawlspace ceiling. The soil may in fact be a problem if he is living on top of ground that is naturally wet. I had an outbuilding in my Seattle house on a slab that was constantly damp for this reason.

    But if Arthur can keep his crawlspace well isolated from the house and passively bring in his dry outside air, I think he's fine.

    For personal context, I have spend a combined 25 years of my life on the west coast; 20 in Oakland and 5 in Seattle. Most of the houses I and the people I knew lived in had vented crawlspaces much like Arthur's and I can't remember a single issue there. I'm not arguing it's the ideal building technique but it seems to work fine in that area. It has nothing to do with luck, it is purely a function of living in a psychrometrically privileged area.

    I live in North Carolina now and I agree that vented crawlspaces are a disaster here...

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |